1. Field of the Invention
The present invention relates to a process of and a system for flouring grains such as wheat grains.
2. Description of Related Art
The grains such as wheat grains each includes an endosperm part which contains starch, gluten-parenchyma and an aleuron layer. The aleuron layer constitutes a surface layer of the endosperm part. The content of starch and gluten-parenchyma, i.e., the content of the endosperm part excepting the aleuron layer, is about 84.0% by weight of the grain, and the content of the aleuron layer is about 7.5% by weight of the grain. The endosperm part is covered with several layers which contain an exosperm layer adjacent to the aleuron layer, a testa layer covering the exosperm layer and a layer of pericarp outside the testa layer. The content of the surface portion of the grain including the layers of aleuron, exosperm, testa and pericarp is about 13.5% by weight of the grain. Further, the contents of the layers of pericarp and testa are about 4% and about 2%, respectively, by weight of the grain. The content of the exosperm layer is of very small weight percentage and almost negligible. The grain also includes embryo, the content of which is about 2.5% by weight of the grain.
In a grain flouring process, the grains are milled into powdery or pulverized grains, and endosperm parts except aleuron parts of the grains, i.e., starch and gluten-parenchyma parts, are separated from the materials of the surface portions of the grains which include pericarp, testa, exosperm and aleuron parts and have a large ash content. The endosperm parts except the aleuron parts, thus separated from the materials of the surface portions of the grains, are recovered as a powder or a flour. However, it is very difficult to completely separate starch and gluten-parenchyma from the materials of the surface portions of the grains. Ordinarily, the percentage (yield) of the resulting product (flour) in which the content of the materials of the surface portions is limited to a comparatively small value is about 75%.
In order to improve the milling efficiency and hence to enhance the yield in the process of flouring wheat grains, a pretreatment of adding moisture to the wheat grains and subjecting the same to conditioning is ordinarily performed. By this pretreatment, the moisture content of the wheat grains is increased from an original value of 11 to 13% to a value of 15 to 16%, the latter value being most suitable for milling. Namely, the wheat grains obtained by being selected, i.e., raw-material wheat grains, are moistened by means of a washer while being washed by the latter. Thereafter, the wheat grains are placed in a raw-material tank and are left at room temperature for 24 to 48 hours to allow water to gradually permeate into the wheat grains (tempering). After this moisture control, the wheat grains undergo conditioning for improving processibility of the grains while the latter are subject to the milling treatment and a secondary treatment, and then the wheat grains are moistened again before the milling treatment.
The conditioning is performed before milling in order to soften inner portions of the grains including starch and gluten-parenchyma and strengthen surface portions of the grains including layers of aleuron, exosperm, testa and pericarp, thereby preventing the surface portions from being damaged and facilitating the surface portions to exfoliate from the inner portions of the grains. However, the effects of addition of moisture based on tempering and conditioning prior to the milling step are reduced, since moisture is released from the inner portions and the surface portions of the grains during the milling step as the grains are broken and pulverized many times by means of roll mills and are passed through a plurality of sifters or sieves and a plurality of purifiers (air separation type purifiers). Exfoliation of the surface portions of the grains from the inner portions thereof thereby becomes difficult, and the surface portions are fractured finely. As a result, the surface portions cannot be completely separated from the inner parts, and hence the qualities of the resulting product are reduced.